How can I subdivide a Surface NURB without affecting the shape?
I am afraid that's not how nurbs are design to work. The way nurbs are suppose to function is to use the least amount of point to describe a curve. Every quarter of a circle is a simple arc and would be suffice to have 3 points in order to describe it.

Is there a reason behind your question that you would care to elaborate? So we may figure out a better way to do if ... if any.